fix(restart): preserve channel metadata across /restart so reply lands in thread

cmd_restart only persisted channel + chat_id across the os.execv boundary, so
when the new process announced "Restart completed" the OutboundMessage had
no Slack thread_ts and the reply fell back to the channel root.

Serialize msg.metadata into NANOBOT_RESTART_NOTIFY_METADATA, restore it on the
RestartNotice, and forward it to OutboundMessage so the completion message
follows the same routing as the original /restart invocation.

Made-with: Cursor

nanobot/command/builtin.py

@@ -28,7 +28,11 @@ async def cmd_stop(ctx: CommandContext) -> OutboundMessage:
 async def cmd_restart(ctx: CommandContext) -> OutboundMessage:
     """Restart the process in-place via os.execv."""
     msg = ctx.msg
-    set_restart_notice_to_env(channel=msg.channel, chat_id=msg.chat_id)
+    set_restart_notice_to_env(
+        channel=msg.channel,
+        chat_id=msg.chat_id,
+        metadata=dict(msg.metadata or {}),
+    )
 
     async def _do_restart():
         await asyncio.sleep(1)